Magnetic tape provides a means for physically storing data. As an archival medium, tape often comprises the only copy of the data. The tape is typically made as thin as practically possible to maximize the length of a tape stored on a tape reel, and thereby maximize the amount of data that can be stored on the tape contained in a single cartridge. A tape drive is used to store and retrieve data with respect to the magnetic tape. While thinner tapes are desired to maximize the amount of tape stored in a single cartridge, thinner tapes are more prone to breakage as a result of mechanical stress or malfunctions of the tape drive. An example of a tape drive is the IBM TotalStorage Enterprise Tape Drive 3592 manufactured by IBM Corporation. Tape drives are typically used in combination with an automated data storage library. For example, the IBM TotalStorage Enterprise Tape Library 3494 manufactured by IBM Corporation is an automated data storage library that may include one or more tape drives and data storage media for storing data with respect to the tape drives.
Tape drives frequently employ DC motors and feedback control systems with motor drivers for operating the DC motors, to produce well controlled motion parameters such as position, velocity, and tape tension. Such control systems are usually very complex, and the feedback control system may compensate for marginal components, hiding latent problems until a catastrophic failure occurs. While the motors rotate, a back EMF (“BEMF”) is produced by the tape drive electric motors. This BEMF voltage is produced because the electric motors generate an opposing voltage while rotating. During deceleration, the stored mechanical energy in the rotating motor is prosegmental to the kinetic energy KE, where, KE=I×w2. Prior art solutions, transfer this stored energy during deceleration to the power supply connected to the tape drive, resulting in the tape drive sending current into the power supply. Because, the energy stored is prosegmental to the square of the tape velocity, a higher performance tape drive has larger BEMF voltage since the BEMF voltage is prosegmental to the rotating speed. Near the beginning and ending of the tape, the motor rotating speeds are highest and result in a larger current driven back into the constant voltage power supply.
BEMF voltage can cause various modes of failure for the tape drive and power supply system. One mode of failure resolved by the present invention involves a BEMF current during a motor deceleration that causes a voltage of a power supply to increase due a lack of sufficient capacitance at the output of the power supply to absorb the incoming BEMF current without increasing the voltage. As a result, the voltage increase may cause the power supply to trip an overvoltage protection circuit that turns off the power supply. For example, a BEMF current flowing into a 12 volt power supply during a motor deceleration can cause an overvoltage protection circuit to turn off the 12 volt power supply as the 12 volt power supply approaches 13.4 volts. What is therefore needed is a technique for reducing, if not eliminating, the BEMF voltage of the electric motor during an electric motor deceleration.